This invention relates to shaft joints between metal and composite material shaft components and, more particularly, to such a shaft joint which is quite effective in transmitting both torsional and axial loads between the metal and composite material components without breaking or cutting the composite fibers. In this connection, the composite material used for a shaft or a component of such a shaft typically includes layers of wound fibers, such as ones of fiberglass, and a relatively high modulus material, such as graphite.
While most shafts for transmitting rotational power typically are made of metal, it often is desirable that such a shaft be made from a composite material to reduce weight and transmitted noise. For example, the final drive shaft of a typical submarine designed to transmit torque to the propeller is often both quite large in diameter, e.g, more than 18 inches, long, e.g, it may be more than 25 feet long. It will be appreciated that if such a giant shaft was made entirely of metal, it would be quite heavy, as well as act as a good transmitter of noise to the outside of the submarine.
While it is common to make the majority of the length of a shaft of a composite material, it is also common to use a selected metal at certain locations which may be subjected to stresses which can be accommodated better by the metal. For example, it is common that propeller shafts be made of a high strength metal at the junction between the shaft and the propeller. Thus composite-to-metal shaft joints must be used. It will be recognized that such a joint must transmit both the axial, bending and torsional loads expected to be encountered.
Those joints that have been designed and used in the past have suffered from various deficiencies. For example, in most existing joints, the attachment (load transmission from composite-to-metal) is accomplished by drilling holes through the composite and installing shear pins. This reduces the strength of the composite in the areas of the drill holes by a factor of two or more. Another approach that has been considered is to glue or epoxy the joint between the composite and metal. This type of joint has its strength limited by the shear strength of the bonding agent. Moreover, it is not uncommon for joints designed in the past to have a diameter that is greater than the diameter of the majority of the shaft in view of the disparate abilities of the composite and metal materials to accommodate the stresses that are to be handled. This can interfere with other structures, cause mass problems, etc. Also, the constraints imposed by the different characteristics of the materials deleteriously interferes with the design choices. The result is that the final design often is less than ideal from some standpoints. For example, most of such joints are more complicated to assemble than desired. It is also difficult to provide such a joint which will transmit all of the expected loads and yet remain dimensionally acceptable.